The invention discloses a method for measuring the average reflection step length of light in a detector. The method comprises: 1) filling a selected detector with a medium to be detected, and arranging a photomultiplier tube, wherein the inner wall of the detector is a high reflection material using diffuse reflection as the main; 2) irradiating in the detector by using a selected light source, and collecting light signals with the photomultiplier tubes; 3) changing the reflection area of the inner wall of the detector, repeatedly performing the step 2), and fitting according to the collected light signals to obtain the effective attenuation lengths [lambda]' of photons in the detector under different reflection areas; and 4) according to the data obtained in the step 3), fitting the point corresponding to the ratio r of the reflection area to the detector inner wall area by using a formula 1/[lambda]'=(1/[lambda])-ln(f)/L-ln(r)/L, and measuring the average reflection step length L of the light in the detector filled with the medium to be detected through fitting. According to the present invention, the method has advantages of simple operation, low error, high precision, novel principle, and strong deep development and application value.